The concept of holography was first described in 1947. However, at the time no coherent light sources existed so holograms were of poor quality and were generally made using mercury arc lamps. With the invention of the laser, holograms improved.
In general, holographic recording generates an interference pattern between a reference wave and an object wave. Unlike two dimensional photography, holographic imaging records both the amplitude and phase of the object wave. Illumination of the hologram with coherent light can recreate both the reference wave and the object wave.
Conventionally, transmission type holography typically uses a photographic plate or photosensitive gelatin film. The plates are typically formed of glass coated with an emulsion of silver halide crystals. Upon exposure to light, the silver ions are partially converted to silver metal darkening the exposed region. As such, the holographic recording process is irreversible. In addition, photoconversion of silver ions to metallic silver generally requires strong light flux or significant exposure times. Over the exposure time, (30 seconds to several minutes), the holographic interference pattern should remain stable to within a quarter wavelength. Vibration isolation tables or short high power laser pulses can be used to achieve this objective.